We have been studying two RNAs, found in the cytoplasm of mammalian cells, which appear to be precursors of two of the most abundant nuclear low molecular weight homodisperse RNAs (C and D, according to the nomenclature of Weinberg and Penman, or U2 and U1, in the nomenclature of Busch and colleagues). The evidence for this precursor-product relationship is mainly their size, their kinetics of appearance and disappearance during pulse-chase experiments, and their oligonucleotide fingerprints. Using non-aqueous methods of cell fractionation we are trying to find out if this cytoplasmic localization is an in vivo phenomenon. We are also studying the methylation of these precursors during their maturation. Another goal is to analyze the particle association of these RNAs during the cell cycle and their chromosome association during the mitotic cycle, as U1 appears to inhibit protein synthesis in a cell-free system (Fed. Proc., 36, 871 (1977), and some of the nuclear low molecular weight homodisperse RNAs seem to specifically associate with chromosomes during mid-anaphase (Nature, 261, 519 (1976).